1. Field of the Invention
This invention relates to a focus detecting apparatus of the T.T.L. type for effecting focus detection by light from an object passed through a photo-taking objective lens, and particularly to a focus detecting apparatus capable of effecting focus detection on the basis of light from objects in a plurality of directions.
2. Related Background Art
Heretofore, a focus detecting apparatus of this type has been designed such that rearwardly of the primary image of an object formed by a photo-taking lens, there is disposed an optical system for re-imaging two secondary images substantially similar to the primary image and the amount of deviation between the relative positions of these two secondary images, i.e., the defocus amount, is detected to thereby accomplish focus detection.
Such a focus detecting optical system is usually disposed so that the re-imaged two secondary images are arranged side by side only in a particular direction (for example, the lengthwise direction of the photographing picture plane). However, the image of the object formed by the photo-taking lens has a luminance distribution in any direction, and the prior-art focus detecting apparatus described above has suffered from the disadvantage that focus detection becomes impossible for an object which does not have a luminance distribution in a direction fit for the focus detecting optical system (for example, the lengthwise direction of the photographing picture plane).
In order to eliminate such disadvantage, there have been proposed and already known several focus detecting apparatuses in which a plurality of pairs of optical systems are disposed so that the directions of deviation between the relative positions of a pair of secondary images may differ from each other. For example, Japanese Laid-Open Patent Application No. 62-95511 discloses a focus detecting apparatus designed such that the pupil is divided symmetrically with respect to the optic axis of a photo-taking lens and a plurality of focus detecting optical systems for forming two secondary images are disposed in different directions to effect focus detection.
Also, Japanese Laid-Open Patent Application No. 63-11906 discloses a focus detecting apparatus in which the pupil is divided symmetrically with respect to an axis differing from the optic axis of a photo-taking lens and a plurality of independent focus detecting optical systems for forming secondary images are disposed, whereby focus detection is possible in a plurality of directions and at a plurality of locations.
However, in the above-described prior-art focus detecting apparatuses in which a plurality of focus detecting optical systems are disposed, a plurality of optical systems for detecting only the deviation between the relative positions of secondary images arranged side by side in one direction are merely disposed in horizontal and vertical directions. Therefore, there is obtained the advantage that as the probability with which there exists an object having a luminance distribution in a direction which is not fit for the focus detecting optical systems becomes lower, the possibility of focus detection becoming impossible becomes smaller, but there is the problem that the focus detecting apparatus becomes bulky.
On the other hand, a focus detecting apparatus which has a plurality of focus detecting optical systems disposed so that the directions of deviation between the relative positions of secondary images may differ from each other and in which the focus detecting optical systems differing in direction are endowed with different functions is also known as disclosed, for example, in Japanese Laid-Open Patent Application No. 63-88511.
This known focus detecting apparatus is designed such that the optical interval between the centers of gravity of the divided areas of the pupil on a photo-taking lens divided by the pupil dividing means of one of the focus detecting optical systems differ from the optical interval between the centers of gravity of the divided areas of the pupil on the photo-taking lens divided by the pupil dividing means of another one of the focus detecting optical systems, and the relative interval between secondary images formed through different pupil areas is changed to thereby make the amount of deviation between the relative positions of the secondary images (the resolving power) for the distance great.
However, in such a focus detecting apparatus having focus detecting optical systems differing in the relative interval between the secondary images, the coefficient for converting the amount of deviation between the relative positions of the secondary images into a defocus amount differs from one optical system to another and as a whole, calculation becomes complicated. Further, in the focus detecting optical system greater in the optical interval between the centers of gravity of the divided areas of the pupil on the objective lens, the non-uniformity of the quantity of light of the secondary images caused by aperture efficiency, i.e., the so-called eclipse, is apt to occur, and such optical system can only cope with photo-taking lenses of great aperture ratio, lacks versatility and moreover, has suffered from the problem that pupil dividing means such as a diaphragm and re-imaging lenses for forming secondary images become extremely bulky.
Furthermore, a focus detecting apparatus in which pupil dividing diaphragms and secondary image forming re-imaging lenses in a plurality of focus detecting optical systems disposed so that the directions of deviation between the relative positions of secondary images may differ from each other are made compact is disclosed, for example, in Japanese Laid-Open Patent Application No. 1-251008. Again in this known focus detecting apparatus, however, the interval between the centers of gravity of a pair of divided areas of the pupil in the longitudinal direction on the exit pupil of the objective lens is made greater than the interval between the centers of gravity of a pair of divided areas of the pupil in the lateral direction. Therefore, the coefficients for converting the amounts of deviation between the positions of the longitudinal and lateral images on the secondary image plane into a defocus amount differ from each other, and the calculation process during focus detection is complicated, and this leads to the disadvantage that high-speed focus detection becomes difficult.